1. Field of the Invention
The present invention relates to an internal combustion engine control device that determines a property of fuel used for combustion in a compression ignition internal combustion engine (e.g., cetane number of light oil) and that controls an operation state of the internal combustion engine according to result of the determination and to a control system using the control device.
2. Description of Related Art
In a compression ignition internal combustion engine, a combustion state in a cylinder changes if a fuel property such as cetane number differs. Therefore, if an operation state of the internal combustion engine is controlled by adapting the control to fuel having a specific fuel property, there is a possibility of occurrence of a problem such as decrease of output torque or increase of a noise. For example, in a case where the operation state of the internal combustion engine is controlled by adapting the control to fuel having a specific cetane number, there occurs a problem of increase of a combustion noise if fuel having a higher cetane number than the specific cetane number is used. If fuel having a lower cetane number than the specific cetane number is used, there occurs a problem of decrease of output torque.
According to Patent document 1 (JP-A-2007-321706), an attention is paid to a phenomenon that a fluctuation amount of engine rotation speed (i.e., output torque increase amount) caused with injection increases as the cetane number increases. Therefore, a control device described in Patent document 1 determines the cetane number based on the fluctuation amount and controls the operation state of the internal combustion engine by variably setting adaptation values in accordance with the determined cetane number.
In a normal operation range, it is common to inject fuel at timing on an advanced side of a compression top dead center. Even if small quantity injection of fuel is performed at such the timing, change in the fluctuation amount corresponding to the cetane number does not appear notably (refer to Tb in FIG. 2). Therefore, the inventors of the present invention examined a scheme of performing the small quantity injection of the fuel at predetermined timing on a delayed side of the compression top dead center (referred to as delayed timing) and determining a fuel property based on output torque increase amount of the internal combustion engine caused with the small quantity injection. In the case where the fuel has a property of low ignitability (i.e., low cetane number), an ignition delay increases and the output torque increase amount becomes a very small value if the small quantity injection is performed at the delayed timing. In the case where the cetane number is high, even if the small quantity injection is performed at the delayed timing, the ignition occurs with the ignition delay similar to the delay in the case where the injection is performed before the compression top dead center and the output torque increase amount becomes a value similar to the value in the case where the injection is performed before the compression top dead center.
When the small quantity injection is performed at the delayed timing and the fuel property is determined based on the output torque increase amount at that time, a variation occurs in a sensing value of the output torque increase amount (i.e., a torque increase amount sensing value). Therefore, from a viewpoint of heightening accuracy of the fuel property determination, it is desirable to perform the small quantity injection multiple times and to obtain the multiple torque increase amount sensing values in order to compensate the variation. However, frequent execution of the small quantity injection causes problems of prolonged execution of an operation state with deteriorated exhaust emission and deterioration of fuel consumption. That is, there is a relationship of trade-off between inhibition of deterioration of accuracy of the fuel property determination and inhibition of deterioration of the exhaust emission and the fuel consumption.